1. Read and remember the words and word-combinations in italic. Tensile strength and deformation
When materials are exposed to forces, such as tension (stretching forces <—□—>) and compression (crushing forces —>□<—), they deform - that is, they change shape. The type of deformation depends on the type of force that is applied.
When a material is subjected to tension, its length will increase by a certain amount. This is called extension or elongation. It is especially important to understand the performance of materials in tension, as their tensile strength (ability to resist tension) is usually lower than their compressive strength (ability to resist compression).
Elasticity and plasticity
Some materials can extend significantly, but still return to their original shape. A material’s ability to do this is called elasticity. Rubber is an example of a very elastic material - it can be elastically deformed to a considerable extent.
If a material has very low elasticity, and is strong, engineers say it is stiff. If a material has low elasticity and is weak, it is described as brittle - that is, it fractures (breaks, due to tension) very easily. Glass is an example of a brittle material.
Some materials can change shape significantly, but do not return to their original shape. We say these materials are plastic. Often, plasticity is described in specific terms. A material that can be plastically deformed by hammering or rolling - for example, lead (Pb) - is malleable. A material that can be drawn out (stretched) into a long length - for example, copper (Cu) - is ductile.
Stages in elastic and plastic deformation
The graph below shows the typical extension behaviour of ductile materials in tensile testing - where a sample bar is subjected to a progressively increasing tensile force.
Points 0-1. The extension of the bar is proportional to the increase in tension. For example, when tension increases by 10%, length increases by 10%.
Point 1. The bar reaches the limit of proportionality. Beyond this point, length begins to increase at a slightly greater rate than tension.
Point 2. The elastic limit is reached. Beyond this point, the bar will no longer return to its original length. In many materials, the elastic limit occurs almost immediately after the limit of proportionality.
Point 3. The bar reaches its yield point. Once it yields, it continues to increase in length, even without a further increase in tension.
Point 4. This is the ultimate tensile strength (UTS) of the material. Beyond this point, a waist (a narrower section) appears at a point along the length of the bar, signalling that it is about to fracture.
Point 5. This is the fracture point, where the bar breaks in two.
2. Complete the sentences using the words in the box. You will need to use one word twice. Look at A opposite to help you.
compression deformation elongation extension tension
A stretching force is called ……………. .
A crushing force is called …………….. .
Extension is also called …………….. .
Tension causes ..….. or .
Tension or compression cause ……. .
3. Match the two parts to make correct sentences. Look at В and С opposite to help you.
-
If a material is stiff
If a material is brittle
If a material is plastic
If a material yields
If a material fractures
If a material is elastically deformed
it is malleable and/or ductile.
it has low elasticity and low tensile strength.
it has low elasticity and high tensile strength.
it has been extended to a point before its elastic limit.
it has been loaded beyond its ultimate tensile strength.
it has been significantly plastically deformed, but not broken.
4. Complete the magazine article about springs using words from A, В and С opposite.
H
ow
are the springs used in car suspension made springy? It sounds like
a silly question, but think about it for a moment. In order for a
spring to compress or extend, then return to its original shape, it
must be
(1) ………
But springs are made from wire, and wire is made from very
(2) …………… metal (often cold drawn carbon steel). When the wire
is manufactured, it is not only stretched beyond its (3) ….. ….. - meaning it will no longer return to its original length - but also beyond its (4) ….. ….. , where significant, irreversible (5) ………. occurs.
The metal from which springs are made has therefore been (6) ….... deformed and, consequently, needs to have its springiness put back.
To do this, once a spring has been formed into a coil, it is tempered – a process in which it is heated and kept at a high temperature for a sustained period. This 'resets' the atomic structure of the metal (partly, at least), so that after tempering, the spring will behave as it should - it can be (7) ….. deformed and will subsequently return to its original shape.
5. Think about a device, vehicle or structure you’re familiar with, and the materials used to make it. What properties do the materials have? Which properties are strengths in this situation? Which properties are weaknesses, and how are these weaknesses overcome?
Unit 9
Material properties 2